add instructions for how to compile libstd with xargo

Author: RalfJung

README.md

@@ -54,6 +54,28 @@ Then, inside your own project, use `cargo +nightly miri` to run your project, if
 a bin project, or run `cargo +nightly miri test` to run all tests in your project
 through miri.
 
+## Running miri with full libstd
+
+Per default libstd does not contain the MIR of non-polymorphic functions.  When
+miri hits a call to such a function, execution terminates.  To fix this, it is
+possible to compile libstd with full MIR:
+
+```sh
+cargo install xargo
+cd xargo/
+RUSTFLAGS='-Zalways-encode-mir' xargo build --target `rustc -vV | egrep '^host: ' | sed 's/^host: //'`
+```
+
+Now you can run miri against the libstd compiled by xargo:
+
+```sh
+cargo run --bin miri -- --sysroot ~/.xargo/HOST tests/run-pass/vecs.rs
+```
+
+Notice that you will have to re-run the last step of the preparations above when
+your toolchain changes (e.g., when you update the nightly).  Also, xargo doesn't
+currently work with nightlies newer than 2017-04-23.
+
 ## Contributing and getting help
 
 Check out the issues on this GitHub repository for some ideas. There's lots that

xargo/Cargo.lock

@@ -0,0 +1,6 @@
+[package]
+name = "miri-xargo"
+description = "A dummy project for building libstd with xargo."
+version = "0.0.0"
+
+[dependencies]

xargo/Cargo.toml

@@ -0,0 +1,2 @@
+[dependencies]
+std = {features = ["panic_unwind", "jemalloc"]}